The present invention relates to an ion trap type mass spectrometer by which wideband auxiliary AC electric fields having the frequency components within the required range are created, then ions having mass-to-charge ratios within the required range are ejected by resonance, and only specific species of ions are analyzed at high sensitivity and high resolution, or only specific species of dissociated ions are analyzed using the tandem mass (MS/MS) method.
The corresponding ion trap type mass spectrometer, as shown in FIG. 2, consists of ring electrode 10 and end cap electrodes 11 and 12 arranged vertically facing one another so as for the ring electrode to be located in between. Hereinafter, the ring electrode and the end cap electrodes are collectively referred to as the ion trap electrodes. Quadruple-pole electric fields are generated in the space between the electrodes by the application of a direct-current (DC) voltage, U, and a radio-frequency (RF) driving voltage VRFCOSxcexa9t, between the electrodes. The stability of the oscillation of the ions that have been trapped in these fields is dictated by the values xe2x80x9caxe2x80x9d and xe2x80x9cqxe2x80x9d in expression (1) below that are given by the size of the apparatus (namely, inner radius xe2x80x9cr0xe2x80x9d of the ring electrode), the DC voltage, U, applied to each electrode, the amplitude, V, and the angular frequency, xcexa9, of the RF driving voltage, and the mass-to-charge ratio, xe2x80x9cm/zxe2x80x9d (kg/coulomb), of the ion.                               a          =                                                    8                ⁢                eU                                                              r                  0                  2                                ⁢                                  Ω                  2                                                      ·                          z              m                                      ,                  q          =                                                    4                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                                  V                  RF                                                                              r                  0                  2                                ⁢                                  Ω                  2                                                      ·                          z              m                                                          (        1        )            
In the above expression, xe2x80x9czxe2x80x9d, xe2x80x9cmxe2x80x9d, and xe2x80x9cexe2x80x9d denote the valence number, mass, and elementary charge, respectively, of the ion. The stability region denoting the range of xe2x80x9caxe2x80x9d and xe2x80x9cqxe2x80x9d in which stability of ion oscillation is given in the ion trap inter-electrode space is shown in FIG. 3.
Since only RF driving voltage VRFCOSxcexa9t is applied to the ring electrode, all ions equivalent to the line of xe2x80x9ca=0xe2x80x9d in the stability region are usually oscillated in the space and trapped between the electrodes. At this time, the point of (0, q) on the stability region differs according to the particular mass-to-charge ratio xe2x80x9cm/zxe2x80x9d of the ion, and each ion is arranged xe2x80x9caxe2x80x9d -axially between xe2x80x9cq=0xe2x80x9d and xe2x80x9cq=0.908xe2x80x9d on the line of xe2x80x9ca=0xe2x80x9d in order of the magnitude of the mass-to-charge ratio, subject to expression (1) above.
In the ion trap type mass spectrometer, therefore, all species of ions whose mass-to-charge ratios fall within a certain range are stably pre-trapped, at which time, the ions oscillate at a different frequency, depending on the xe2x80x9cm/zxe2x80x9d value. This characteristic is utilized for auxiliary AC electric fields of a specific frequency to be superimposed in the ion trap inter-electrode space, and only the ions that resonate with the auxiliary AC electric field therewith undergo mass separation.
Of all ions in the specimen, only those to undergo mass separation are sequentially scanned in terms of mass (mass scan analysis) to obtain a mass distribution chart (mass spectral chart) of all ingredients in the specimen. At this time, the quantity of ions which can be trapped in the ion trap inter-electrode space is realistically limited because increases in the quantity of ions trapped increase the effects of the space charge and thus reduce the analyzing performance of the apparatus.
Therefore, when the mass range (xe2x80x9cm/zxe2x80x9d range) of undesired ions, or ions not to be analyzed, is known or when the mass range (xe2x80x9cm/zxe2x80x9d range) of the necessary ions, or the ions to be analyzed, is known, all species of undesired ions can be ejected from the ion trap inter-electrode space before the ions in the specimen undergo mass spectral analysis.
Once all unnecessary ions have been ejected from the specimen, the number of necessary species of ions trapped in the ion trap inter-electrode space will correspondingly increase and thus analytical sensitivity will increase. Also, when only ions of a specific mass number (namely, parent ions) are trapped, dissociated, and undergo tandem mass spectral analysis (MS/MS analysis) to obtain the mass distribution of the dissociated ions, the quantity of parent ions trapped can be increased by ejecting all non-parent ions as undesired ion species. In addition, the creation of dissociated ions from non-parent ions can be avoided.
Since this MS/MS analytical method enables the acquisition of further detailed information on the molecular structure of specific ions, the MS/MS analytical function has come to be among the most important functional requirements of a mass spectrometer in recent years.
Various methods of eliminating all undesired ions whose xe2x80x9cm/zxe2x80x9d values fall within the required range have been developed up to now. For example, a method of ejecting such ions by applying wideband signals to the ion trap electrodes during the mass spectrographic scanning period is disclosed in U.S. Pat. No. 4,761,545.
Also, methods in which all undesired having their own oscillational frequencies falling outside the specified band are ejected by applying a frequency band-pass filter to noise waveforms are disclosed in U.S. Pat. No. 5,134,286 and Japanese Application Patent Laid-Open Publication No. Hei-7-509097.
In the above-mentioned examples, although the two methods differ in that whether they use a frequency band-pass filter, such a wideband auxiliary AC voltage as shown in expression (2) below, is applied to the ion trap inter-electrode space.                                           V            FNF                    =                                    ∑              i              n                        ⁢                                          v                i                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              (                                                      ω                    i                                    +                                      φ                    i                                                  )                                                    ,                                            ω                              i                +                1                                      -                          ω              i                                =          Δω                                    (        2        )            
Although these methods have heretofore been proposed for phase control between frequency components, since constant values are set for the amplitude, Vi, of each frequency component and the angular frequency division width, xcex94xcfx89, between frequency components, no control has been provided as to the wideband auxiliary AC voltage, VFNF, or as to the amplitude, Vi, of each frequency component or the frequency division width, xcex94xcfx89, between frequency components according to the RF driving voltage value VRF.
Under the prior art, when wideband auxiliary AC voltages having interspaced different frequencies within a frequency range equivalent to the resonance frequency of undesired ions are applied between ion trap electrodes in order to eject these undesired ions resonantly from the ion trap inter-electrode space, constant values are usually set for the auxiliary AC voltage amplitude, Vi, of each frequency component and the frequency division width, xcex94xcfx89, between frequency components.
In the case that wideband auxiliary AC electric fields are generated this way by voltage application, there occurs the problem that even the undesired ions within the specified range are not effectively removed by resonant ejection and remain between the ion trap electrodes.
Other problems also occur. That is to say, because of low resolution in mass separation of undesired ions and the desired ions to be analyzed, the undesired ions whose frequencies are close to those of the desired ions cannot be ejected or the desired ions are ejected along with the undesired ions.
The purpose of the present invention is to supply the ion trap mass spectrometry and ion trap mass spectrometer that enable highly efficient release of undesired ions having a wide range of xe2x80x9cm/zxe2x80x9d values, and the separation of undesired ions and the desired ions at high resolution.
The present invention for fulfilling the purpose described above relates particularly to an ion trap type mass spectrometer that comprises:
a ring electrode,
a pair of end cap electrodes facing one another so as to arrange said ring electrode between said end cap electrodes,
a radio-frequency power supply for applying to said ring electrode and said end cap electrodes a radio-frequency voltage that generates a radio-frequency electric field in the space formed between said two types of electrodes,
an ion generator for creating ions inside said inter-electrode space or creating ions outside said space and introducing the ions thereinto,
a trapper for trapping created ions in said inter-electrode space,
an alternating-current (AC) electric field generator for generating in said space a wideband auxiliary AC electric fields of different frequencies within the frequency range required for resonant release only of the ions within the required mass-to-charge ratio range among all created ions, and
a detector by which the ions that have been trapped in said space are sequentially separated in terms of mass according to the particular mass-to-charge ratio of the ion and then after the trapped ions have been emitted from the space, the mass of these ions are detected;
wherein the strength of wideband auxiliary AC electric fields comprising the frequency components within the required frequency range mentioned above is varied according to the strength of the radio-frequency electric field generated in the space.
More specifically, the present invention fulfills the aforementioned purpose by: (1) Changing the wideband auxiliary AC voltages of different frequencies within the required frequency range according to the applied RF driving voltage.
(2) Changing the amplitude, Vi, of each frequency component at a wideband auxiliary AC voltages according to the frequency of the frequency component or range of the frequency component at the wideband auxiliary AC voltage.
(3) Changing the frequency division width, xcex94xcfx89i, between the adjoining frequency components of a wideband auxiliary AC voltage according to either the frequency of the frequency component or the xe2x80x9cm/zxe2x80x9d value of the ion at which the frequency of the frequency component becomes equal to the oscillation frequency of that ion in the ion trap inter-electrode space.